(1) Field of the Invention
The present invention relates to a light receiving amplification circuit used primarily in a pickup of an optical disc.
(2) Description of the Related Art
In recent years, optical disc apparatuses able to reproduce and record information have become widespread.
In such an optical disc apparatus, the optical power of a laser differs between when recording and when reproducing. Therefore, a light receiving amplification circuit utilized in signal readout of an optical disc must switch the gain resistor of the operational amplifier and keep the output voltage within the output dynamic range of the amplification circuit when converting a light signal to an electrical signal and further amplifying that electrical signal.
One example of a light receiving amplification circuit which has a gain switching function is disclosed in, for example, Japanese Patent No. 3142214. This light receiving amplification circuit is shown in FIG. 1. In this example, a gain resistor in the operational amplifier which amplifies the output of a light receiving element 801 connected in an anode common is switched with a PNP transistor 807.
The output of the light receiving element 801 is connected to the input of an operational amplifier circuit made up of NPN transistors 803 and 804, and two gain resistors 805 and 806. The gain resistor 806 is directly connected to the output of the operational amplifier, and the other gain resistor 805 is connected to the collector of the PNP transistor 807. An emitter of the PNP transistor 807 is connected to the output of the operational amplifier circuit.
A base of the PNP transistor 807 is connected to a constant current source 810, and the on/off of the PNP transistor 807 is controlled with a switch 811.
In the abovementioned circuit configuration, because the PNP transistor 807 is controlled to be off during reproduction where the laser's optical power is low, the gain of the operational amplifier circuit is set by the gain resistor 806 only.
On the other hand, the PNP transistor 807 is on during recording where the laser's optical power is high, and the gain of the operational amplifier is set by a parallel connection of the gain resistor 805 and the gain resistor 806.
In this manner, the operational amplifier operates with a high gain value during reproduction when the photoelectric current inputted to the operational amplifier is low, and a low gain value during recording when the photoelectric current inputted to the operational amplifier is high. Therefore, stable operation, in which the output of the operation amplifier is not saturated even if the laser power changes drastically, is possible.
However, in the abovementioned conventional configuration, in the case where the PNP transistor 807 is on, there are situations where, in addition to the voltage arising in both ends of the gain resistor 805, a saturation collector-to-emitter voltage Vce (sat) of the PNP transistor enters in between the input terminal and the output terminal of the operational amplifier. Therefore, there is a problem in which the gain of the operational amplifier differs from the designed value and desirable operational characteristics cannot be obtained.